I "think" they are made like that to clear the gas block on your rifle's bigger brother and don't use an end cap.
It is designed to be a free float forend so my guess is that nothing is supposed to go there that could touch the barrel.
Unfortunately without the gas block it may look a little odd to some.
I would put one of those long barrel shrouds Tacticool22 or Belt Fed makes to fill up a good part of that empty space and give it a bit more sinister look.